"cruel," late 13c., possibly late Old English, perhaps from Old French fel "cruel, fierce, vicious," from Medieval Latin fello "villain" (see felon). Phrase at one fell swoop is from "Macbeth." Related: Fellness.

"rocky hill," c. 1300, from Old Norse fiall "mountain," from Proto-Germanic *felzam- "rock" (source also of Old High German felisa, German Fels "stone, rock"), from PIE root *pel(i)s- "rock, cliff." Old High German felisa "a rock" is the source of French falaise (formerly falize) "cliff." Now mostly in place-names, such as Scafell Pike, highest mountain in England.

"skin or hide of an animal," Old English fel "skin, hide, garment of skin," from Proto-Germanic *felnam- (source also of Old Frisian fel, Old Saxon fel, Dutch vel, Old High German fel, German fell, Old Norse fiall, Gothic fill "skin, hide"), from PIE *pel-no-, suffixed form of root *pel- (3) "skin, hide." Related: Fellmonger.